Add 3 cells to prod-lon
Add 3 cells to prod-lon

The memory usage in prod-lon has been climbing, and we're now below our
threshold again.

Judging from this graph:

https://prometheus-1.london.cloud.service.gov.uk/graph?g0.range_input=2w&g0.expr=rep_memory_capacity_pct%3Aavg5m&g0.tab=0

Last time we increased our scale by 3 instances we got an extra 23%
rep_memory_capacity_pct. We're now running at 23%, which is below our
threshold of 33%. Increasing the cell instances by 3 should give us
another 20% or so, which should take us above the threshold again.

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf